Combustion chambers and reactors are used in chemical product production, aircraft, and rocket engines. Usually, such devices as furnace boilers are an integral part of everyday life, and am taken for granted to work safely to supply heat, power, and propulsion. Sometimes these devices fail as a result of rapid uncontrolled combustion or reaction, resulting in serious consequences for the operating system.
Gaseous fuels such as methane, ethane, propane, and the like, produce deflagrations with pressures about 3 to 5 times the mean combustion pressure. Solid fuels, such as coal dust, wood particles, wood flour, and other solid organic fuels, will produce over pressures of 5 to 6.7 above mean (usually one atmosphere) combustion pressure. Some fuels, such as hydrogen and acetylene, may detonate and produce over pressures of 20 above the normal combustion pressure. Common industrial fuels, such as water gas, coke oven gas, and the like, with high amounts of hydrogen and carbon monoxide may detonate and produce high over pressures of more than 10. Some liquid fuels, such as gasoline and fine mists of kerosene, readily deflagrate and produce over pressures of more than 3. Also, obstructions and complexity may promote a deflagration to a detonation transition (DDT) where extremely high over pressures are possible. Relatively benign fuels, in the presence of added oxygen, may exhibit a tendency to form a DDT. The possible DDT is usually not possible to be incorporated into the design since the pressure vessel would make the combustion chamber too expensive to be practical.
Rocket engine flame-out and re-ignition can be very fast, with the possible structural failure of the engine. When flame-out is experienced in an aircraft engine, the aircraft loses altitude rapidly and may crash if flame-out occurs close to the ground, on takeoff, or in a climb over a mountain range. Flame-out of one engine of a two-engine aircraft can cause uncontrollable yaw, and roll, leading to a crash and loss of the aircraft.
In the home, the gas furnace may not have been properly cleaned and the center flame holders work fine permitting the furnace to be started, but the side flame holders may be partially clogged. If satisfied, the thermocouple used to monitor the pilot flame indicates sufficient emf to permit the main gas valve to open and allows full gas delivery. After some short period of time, the flue will fill with unburned gas, and may ignite. If the furnace is propane or LPG, the possible explosion may seriously damage or destroy the house.
In the chemical process industry, an explosion from a reaction interruption and uncontrolled process restart, can result in equipment loss. In each of these, and other scenarios, the consequences of a flame-out in a reaction chamber or combustor can progress to an explosion.
In some cases described, the explosion may be mild, with weak subsonic deflagration, and simply be accompanied by some noise and possibly some smoke. In other situations, the results can be more serious where the deflagration to a detonation transition (DDT) occurs, due to extremely high over pressures. Mild explosions may damage only the combustor where the events are simple deflagrations resulting from the combustion waves moving through the combustible mixture at subsonic (weak deflagration) to sonic speeds (strong deflagration). Usually for a strong deflagration, the pressure increase is about 3 to 5 times the operating pressure.
In other cases, the event may destroy, not only the combustor or reactor but, the entire vehicle and/or factory/industrial complex, as well. These events are usually detonations, where the combustion wave proceeds at supersonic speeds up to about Mach 4 to 6, and the pressure increases by a factor of 20 or more.
These problems may be eliminated, or at least minimized, by detecting the flame-out quickly and closing the fuel valve and possibly restarting the engine or process only in a predetermined and controlled fashion.
The science of flame-out detection or flame establishment was created years ago and a number of systems have been developed to detect flame-outs and stop fuel supply to combustion chambers prior to possible deflagrations becoming detonations. Currently available flame-out detectors are too slow for rocket chambers and high pressure reactors and/or are difficult to install. Also, the available flame-out detectors lack reliable quick detection capabilities. Fast speed, 20 to 200 ms (milliseconds), and very high reliability are needed for use in gas turbines of aircraft.
Most commercial flame-out detectors are slow (about 1.0 second or longer) and are not completely reliable, or the detectors only sample a small part of the reacting volume. In some prior art systems, quickness of detection may be achieved but reliability is poor and false indications are frequent. The prior art flame-out detectors include (1) the use of one or more thermocouples immersed in the flame; (2) ultraviolet (UV) detectors to "stare" at the flame; (3) photo multipliers (PM) to "stare" at the flame; (4) photodiodes to "stare" at the flame; infrared detectors (IR) to "stare" at the flame; (6) acoustic detector (combustor housed) based on a standing wave at 30 Hz; and (7) fiber optic detector by Borg.
The disadvantages in all of these systems is that they, (or the sensing optical fiber) have to be located inside the combustor. The environment inside the combustor is very hostile and, for accuracy, the detector must be located in a cool region or be cooled. Additionally, the noise and vibration level in combustors is very high and may cause premature failure of even the best device.
The thermocouple is slow, typically as slow as 0.20 seconds, with total response time of 1 to 2 seconds. If the temperature is above 3400.degree. F., the only thermocouple available is Iridium-Rhodium which has serious hysteresis problems. If the temperature is above 4000.degree. F., then thermocouple use is not possible. Rocket combustors operate at temperatures of 5000.degree. F. to over 6000.degree. F.
UV detectors and their associated circuits are slow, with typical response time to close or activate being 1 to 4 seconds. This makes their use in high flow and high pressure combustors almost useless.
PM tubes are sensitive to vibration and heat. Thus, these detectors must be used with fiber optics. The high sensitivity may also be a problem since some flames at high pressure are so bright that filters must be used. The flame-out may be at a low light level such that the PM with a filter could not detect the event. The speed of the PM may be a problem since flame-out is not an instantaneous process. The flame may be out in some regions of the combustor but still reacting in others, thus, the aiming area or volume is critical to the use of the PM.
The silicon photo diode is rugged and not so sensitive as the PM, but the aiming and small view angle is still a problem. Another problem is that, in a large combustor, a flame-out may have occurred but it takes considerable time for the hot combustion products to leave the combustor, e.g. 300-500 milliseconds. Thus, the aiming area or volume is critical.
The IR detectors are attractive but the same problem of decaying combustion and time to leave the chamber slow the response. Hot combustor walls may also be a problem since they will continue to give a strong signal long after the cessation of combustion.
The acoustic detector currently used in the large combustor of the NASA Langley 8-Foot High Temperature Tunnel (HTT) is reasonably fast and gives no false indications of flame-out. However, for good response, the pressure sensor must be located within the combustor itself. Any changes in the hostile environment within the combustor affects the accuracy and reliability of any sensor element located therein. An additional problem arises when a damping plate and resonator are added to the combustor wherein the noise levels decreased dramatically, and the 30 Hz was eliminated. Thus, the simple standing wave type approach needs to be improved to make the technique universal, and it would be desirable to have the detector outside the combustor in a benign location that is readily reached and easy to use. This example is specific but the logic is universal.
The fiber optic detector by Borg (S. E. Borg et al, "An Optical Flameout Detection System for NASA Langley's 8-Foot High Temperature Tunnel"; May 1993; Instrument Society of America), is reasonably fast but the fiber optic must be inside the combustor and look through the fuel injector at the correct location for accurate functioning. Also, the optics must be kept clean. If the fuel injector is changed or if the optics are dirty, the unit will not function.
There is, thus, a definite need in the art for a quick, reliable, easy to install, flame-out detector system.